Pressure-sensitive adhesives (PSAs) have been known for decades. Pressure-sensitive adhesives are adhesives which even under a relatively weak applied pressure permit a durable connection with the substrate and which after use can be detached from the substrate again substantially without residue. Pressure-sensitive adhesives are permanently pressure-sensitively adhesive at room temperature, in other words having a sufficiently low viscosity and a high tack, meaning that they wet the surface of the respective substrate under even low applied pressure. The adhesive bonding capacity of the adhesives derives from their adhesive properties, and their redetachability from their cohesive properties. Different compounds are contemplated as a basis for pressure-sensitive adhesives. Main groups include natural rubbers, synthetic rubbers, and acrylates.
Pressure-sensitive adhesives based on natural rubber have been known for a long time.
Natural rubber possesses very good cohesive properties. However, the poor aging resistance and weathering stability constitutes a problem.
CA 698 518 describes an operation for achieving production of a composition through addition of high plasticizer fractions and/or simultaneously strong mastication of the rubber. Although by this method it is possible to obtain PSAs with extremely high tack force, the relatively high plasticizer fraction or else the severe degradation of the molecular structure of the elastomer to a molecular weight average of Mw≦1 million means that there are limitations on the achievability of the user-compatible shear strength, even with relatively high crosslinking subsequently.
The use of polymer blends in which not only nonthermoplastic natural rubber but also block copolymers are used, in a ratio of approximately 1:1, is essentially an unsatisfactory compromise solution, producing neither high shear strengths when the self-adhesive tapes are employed at relatively high temperatures, nor significant improvements over the properties described in the patent.
JP 07 324 182 A2 describes a multistage method in which a double-sidedly adhesive tape has a pressure-sensitive adhesive layer based on an acrylic resin adhesive, and a second layer comprising a blend of isoprene-styrene elastomer, natural rubber, and nonreactive hydrocarbon resin (Arkon P 100). This tape serves as a carpet laying tape, which is likewise not subject to any exacting requirements with regard to shear strength at higher temperatures.
The use of nonthermoplastic elastomers is additionally described in JP 95 331 197, in which an isocyanate-reactive natural rubber (polyisoprene grafted with maleic ester) having an average molecular weight of below 1 million with aliphatic, nonreactive hydrocarbon resins is used, which is crosslinked with blocked isocyanates (for example, Desmodur Conn.), the mixture being precrosslinked at 150° C. for five minutes and, following subsequent coating onto PET film, being cured at 180° C. for several minutes (for example, 15 minutes). This procedure makes it clear how complicated it is to achieve postcrosslinking if the natural rubber is subjected to excessive degradation during the production process.
Patent application JP 95 278 509 protects a self-adhesive tape in which the natural rubber is masticated to an average molecular weight of Mw=100 000 to 500 000 in order to provide a coatable, homogeneous mixture with hydrocarbon resins, rosin/rosin-derivative resins, and terpene resins, which have good processing properties at between 140° C. and 200° C. and at a coating viscosity of 10 to 50×103 cps, but which require an extremely high subsequent EBC dose (40 Mrad) in order to ensure the shear strength that is necessary for service. For carrier materials such as impregnated and/or sized papers and also fabric carriers based on rayon and the like, the system is poorly functional, since the necessarily high radiation doses entail significant carrier damage.
The use of exclusively nonthermoplastic rubbers as elastomer component in the formulation of pressure-sensitive adhesives, with the existing cost advantage possessed by natural rubbers, for example, over the standard commercial block copolymers, and the outstanding properties, more particularly the shear strength, of the natural rubber and of corresponding synthetic rubbers, is also set out comprehensively in patents
WO 94 11 175, WO 95 25 774, WO 97 07 963, and, accordingly, U.S. Pat. No. 5,539,033, U.S. Pat. No. 5,550,175.
Described in this context are the additions that are customary in PSA technology, such as tackifier resins, plasticizers, and fillers.
The production method disclosed in each case is based on a twin-screw extruder which, with the selected process regime involving mastication of the rubber and subsequent staged addition of the individual additives, with a corresponding temperature regime, allows compounding to take place to form a homogeneous PSA blend. Described comprehensively is the step of masticating the rubber, which precedes the production process itself. This step is necessary for and characteristic of the method selected, since with the technology selected there it is vital for the subsequent incorporation of the further components and for the extrudability of the ready-blended composition. Also described is the infeed of atmospheric oxygen, as recommended by R. Brzoskowski, J. L. and B. Kalvani in Kunststoffe 80 (8), (1990), p. 922 ff., in order to accelerate mastication of the rubber.
This process makes the subsequent step of electron beam crosslinking (EBC) vital, as is the use of reactive substances as EBC promoters in order to achieve an effective crosslinking yield.
Both method steps are described in the stated patents, but the EBC promoters selected also tend toward unwanted chemical crosslinking reactions at elevated temperatures. This limits the use of certain tackifying resins. Pressure-sensitive adhesives based on polyacrylates have also been known for a long time. Polyacrylates possess very good adhesive properties, but polyacrylates are inferior to other compounds in the area of cohesion.